Conception calculator: when did I conceive?
Conception most likely around Friday, August 21, 2026
Due date − 266 days. Because sperm can survive up to five days, the sex that led to conception could have been any time from Sun, Aug 16 to Sat, Aug 22. Due dates from early scans are accurate to about a week, so treat this as a window, not an exact day.
🔒 Calculated in your browser. Nothing you type is sent or stored. Estimates only: bodies vary.
A conception calculator counts back 266 days (38 weeks) from your due date to find your most likely conception date. Because sperm can live up to five days, the sex that led to the pregnancy could have happened up to five days before that date, or the day after. Enter your due date above to see your conception date and window.
How the conception calculator works
Your due date is set 280 days after the first day of your last period, which assumes you ovulated and conceived about 14 days later. Working backward, conception is due date − 266 days. The calculator then widens that to a seven-day window, because conception and sex aren't always the same day.
- Enter the due date from your midwife, doctor or dating scan (a scan-based date is best).
- Read your most likely conception date.
- See the window of days when sex could have led to the pregnancy: from 5 days before that date to 1 day after.
Why it's a window, not a day
Sperm can survive in the body for up to five days, waiting for an egg. The egg lives for about 12 to 24 hours after ovulation. So sex on a Monday can lead to conception on Friday. That's also why the Wilcox study found the fertile window is six days long, ending on ovulation day.
A worked example
Your dating scan puts your due date on June 20. Counting back 266 days gives a likely conception date of September 27 the year before. The sex that led to the pregnancy most likely happened between September 22 and September 28.
Conception date by due date
Likely conception dates for a due date on the 1st of each month (a year that crosses February 29 shifts the date by one day).
| Due date | Likely conception (previous year unless noted) |
|---|---|
| January 1 | April 10 |
| February 1 | May 11 |
| March 1 | June 8 |
| April 1 | July 9 |
| May 1 | August 8 |
| June 1 | September 8 |
| July 1 | October 8 |
| August 1 | November 8 |
| September 1 | December 9 |
| October 1 | January 8 (same year) |
| November 1 | February 8 (same year) |
| December 1 | March 10 (same year) |
Calculated in your browser, nothing stored. It assumes ovulation about 14 days before your period was due, which isn't true for everyone.
How accurate is a conception date?
It depends on how your due date was set. A due date from a first-trimester scan is usually accurate to within about 5 to 7 days, so your conception date carries the same margin. A due date worked out only from your last period can be further off, especially with long or irregular cycles, because ovulation may have happened much later than day 14.
| How the due date was set | How precise the conception date is |
|---|---|
| IVF transfer date | Exact: you know the fertilization date |
| Known ovulation day (LH test or temperature) | Within a day or two |
| First-trimester dating scan | Within about a week |
| Last period only, regular cycles | Within about a week, sometimes more |
| Last period only, irregular cycles | Could be off by two weeks or more |
Can a conception calculator tell me who the father is?
Not reliably. If two dates of sex were more than a couple of weeks apart, the dates can sometimes point clearly to one. If they were within about 10 days of each other, the margin of error is too big. A prenatal DNA paternity test, available from about 7 weeks, is the only way to know for sure. If this question weighs on you, it's okay to ask your midwife or doctor for support in confidence.
Looking back at dates can stir up a lot, from excitement to worry. If you know your ovulation day already, the due date calculator based on ovulation works in the other direction. And if you're tracking your next cycle, our ovulation calculator shows your fertile window ahead of time.
Book your first pregnancy appointment as soon as you have a positive test. Get urgent care for heavy bleeding, severe or one-sided tummy pain, shoulder-tip pain or fainting.
Frequently asked questions
How do I calculate when I conceived?
Subtract 266 days from your due date. If your due date is May 1, you most likely conceived around August 8 of the year before.
How many weeks pregnant are you at conception?
About two weeks. Pregnancy is counted from the first day of your last period, roughly two weeks before you ovulate and conceive.
Can you conceive days after having sex?
Yes. Sperm can survive up to five days, so conception can happen several days after sex if you ovulate later.
Is the conception date the same as the date of sex?
Not always. Conception happens when sperm meets the egg, on ovulation day or the day after, which can be up to five days after sex.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. NEJM (1995)
- NHS: Pregnancy due date calculator
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
Keep reading
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